1. Field of the Invention
The present invention relates to an electrically-conductive polymer used for electronic devices such as batteries, capacitors, diodes, indicator elements, and sensors, a production method of the polymer, and a solid-electrolytic capacitor using the polymer as an electrically conductive layer.
2. Prior Art
In general, an electrically-conductive polymer of this sort is produced by adding a dopant to a polymer produced by chemically-oxidative polymerization of a chemical compound having .pi. electrons.
For example, in the Japanese Non-Examined Patent Publication Nos. 64-74711, 64-74712, and 64-74713, which were published in 1989, typical examples of the conventional production methods of a solid-electrolyte capacitor are disclosed. With these conventional methods, an electrically-conductive polymer film is formed on an oxide film of a film-formation metal by chemical polymerization of a proper monomer using an oxidizing agent. The electrically-conductive polymer film may be formed by electrolytically oxidative polymerization of a proper monomer. This electrically-conductive polymer film, which is produced by a chemical polymerization using an oxidizing agent or electrolytically oxidative polymerization, is in a highly oxidized state where anions are doped into the polymer. Thus, the electrical conductivity tends to deteriorate due to heat because of its structural change and therefore, it is difficult to maintain the initial electrical conductivity stably over a long period of time. This fact has been known well.
In recent years, in the field of solid-electrolytic capacitors, it has been noticed that the electrically-conductive polymer is superior in electrical conductivity when compared to such a substance as manganese dioxide that has been widely used as a solid electrolyte. Thus, various active researches and developments have been made to improve the capacitor characteristics including the solution of the above problem of electrical conductivity deterioration at a high temperature.
For example, in the Japanese Non-Examined Patent Publication Nos. 4-48710 published in 1992, 2-58817 published in 1990, and 5-217808 published in 1993, improved capacitors are disclosed, each of which uses an electrically-conductive polymer containing a suitable aromatic sulfonic acid or its salt. These capacitors were developed for the purpose of preventing the electrical conductivity of an electrically-conductive polymer film from lowering at a high temperature. With the conventional electrically-conductive polymer containing an aromatic sulfonic acid or its salt, the aromatic sulfonic acid or its salt serving as a dopant has a high molecular weight and therefore, this dopant is difficult to be desorbed or released from the main chain of the electrically-conductive polymer even at a high temperature. As a result, this electrically-conductive polymer has a better heat resistance as compared to that of the other electrically-conductive polymers.
In the Japanese Non-Examined Patent Publication Nos. 5-129162 published in 1993 and 3-35516 published in 1991, improved capacitors are disclosed, each of which uses an electrically-conductive polymer containing an anion of a proper proton acid. In general, an electrically-conductive polymer containing an anion of a proton acid has a so-called "self-repairing function" and a low electrical-impedance characteristic in a high-frequency region. The "self-repairing function" is a function or capability that reoxidizes a defective portion of an oxide film of a film-formation metal in a solid-electrolyte capacitor to thereby decrease a leakage electric-current of the capacitor.
Especially, the electrically-conductive polymer disclosed in the Japanese Non-Examined Patent Publication No. 3-35516 is soluble in an organic solvent in the state where the dopant is desorbed or removed, and it develops electrical conductivity by means of doping a proton acid having a pKa value of 4.8 or less thereinto. This polymer makes it possible not only to decrease the electrical impedance of a capacitor in a high-frequency region but also to enable the formation of an electrically-conductive layer by means of a coating process Therefore, this polymer facilitates simplification of the production processes of the capacitor.
Any one of the proton acids mentioned in the above Japanese Non-Examined Patent Publication Nos. 5-129162 and 3-35516 is a monovalent proton acid, which emits a single proton per molecule.
In the Japanese Non-Examined Patent Publication No. 1-100911 published in 1989, a method of forming an electrically-conductive polymer layer on an oxide film by a two-step polymerization technique is disclosed. With this method, a solution containing an oxidizing agent such as bichromic acid, persulfuric acid, and permanganic acid is used at the first stage of polymerization, thereby forming a part of the electrically-conductive polymer layer by electrolytic polymerization. Then, at the second stage of the polymerization, the remaining part of the electrically-conductive polymer is formed by electrolytic or chemical polymerization.
When the second stage of the polymerization is performed by electrolytic polymerization, a liquid electrolyte containing R.sub.4-x MH.sub.x or R.sub.3 M' is used, where R is an alkyl group of C.sub.1 to C.sub.10, or an aryl group such as phenyl and alkylphenyl, M is N, P, or As, and M' is O or S, and x is 0 or 1. On the other hand, when the second stage of the polymerization is performed by chemical polymerization, an acid such as persulfuric acid, perboric acid, and permanganic acid, or its alkali or ammonium salt is used as an oxidizing agent.
The two-step polymerization technique disclosed in the Japanese Non-Examined Patent Publication No. 1-100911 makes it possible to uniformly form an electrically-conductive polymer layer on the underlying oxide film without damage of the oxide film. Further, this technique enables uniform formation of an electrically-conductive polymer layer even on an fine-etching-induced pit area in an aluminum electrolytic capacitor, for example.
As described above, in the field of solid-electrolytic capacitors, a large number of researches and developments have been performed to improve the characteristics of the electrically-conductive polymer serving as a solid electrolyte.
An electrically-conductive polymer containing an aromatic sulfonic acid with a high molecular weight or its salt makes it possible to prevent its electrical-conductivity deterioration at a high temperature, as disclosed in the Japanese Non-Examined Patent Publication Nos. 4-48710, 2-58817, and 5-217808. However, this polymer has a problem that the ability of maintaining the initial electrical conductivity for a long period of time is not satisfactory in the typical range of the service temperature and humidity for an electronic device, especially in a high-temperature atmosphere.
Specifically, with the electrically-conductive polymer containing an aromatic sulfonic acid with a high molecular weight or its salt, the high-temperature electrical-conductivity deterioration is able to be suppressed, which results from a main cause (a) that the dopant is desorbed or released from the main chain of this electrically-conductive polymer However, the main-chain cleavage of this polymer due to oxygen-addition reaction, which is another main cause (b) for the high-temperature electrical-conductivity deterioration, cannot be suppressed. Therefore, it is difficult for this polymer to resist the deterioration with time due to the latter cause (b) to maintain the initial electrical conductivity for a long period of time